Manufacture of sponge rubber



Patented June 20, 1950 UNITED STATES PM orrrcr James UL- Mann,,Kearny, N. J .',,assign0r to United States Rubber Company, New York; N. Y., a corporation of New Jersey No Drawing; Application-June24, 1949.- SeriaLNo. 101,250.-

3 Claims; (Cl. 18 53) This invention relates to the manufactureof sponge rubber from latex andmore, particularly to heat-setting, ahlatexfroth by means. of a low frequencybalanced. alternating, electric current.

The heat-settingof. shaped. latex. froths. 0011-. taining heat-setting, agents bypassing low frequency, balanced. alternating, electric currents from electrodes through the. froths to generate the desired amount of heat in the frothtoset or gel the same,.prior tosubjectingthe gelled froth to conventional vulcanizingconditions, is, known. The use of various. metals as the electrodes,. gives rise to unsightlypitting or discoloration, tor. both, of the surface of the sponge rubber molded against thelelectrodes. For instance, aluminum, which is commonly used for molds, gives .very .bad. pitting; ironor.stainlesssteel, whichais alsoused for molds, gives pitting accompanied by, brown discoloration; and tin, which has been suggested for electrodes in lowv frequency alternating current heating, leaves yellow. and. black. spots. on. the surface of the finalsponge rubber article.

I have found that if the surface of the electrode in contactwith the froth in such low frequency balanced alternating current heatingis. made of. zinc or cadmium, the surface of the sponge article which was in contact with the zinc or cadmium electrode duringits setting or gelling will be free from pits and discoloration.

Incarrying out the present invention. aheatsettable latex. froth: is introduced into a-mold by, forming themfroth in the mold as Joy -chemical. blowing byv evolving. ages in the latexin the mold; or by.chemically blowingthelatex in-bulk or mechanicallywhipping a body of llatex intoas froth and pouring; the. froth into individual stactionary or traveling-mold: or by pouring'or otherwise introducingthe: froth into atraveling; con-,- tinuous mold. Such molds'will contain two'electrodes; at least one of which will be in contact with a portionv of the outer'surface of :thersponge: article being molded;v The secondelectrode could. be embedded in the body of the frothbeing molded andremoved aftergellin-g of the froth;.butigenerally each electrode will be comprise'd in' onerofi' the twoparts of a two-part mold, usually the bodypart and the cover part. Forexample, the bot-'- tom and top plates of a'box type mold may-be composed of metal with the sides made of 'woodr or other insulating material, or the mold may be all made of wood-withmetal plates secured to the opposite inner faces of the top and bottom sections, the metal plates, of course, being made the electrodes for the low frequency electric current to be passed through the froth. Also, the

body' of the mold may be of metal 'andcomprise. one electrodaand the cover of, the moldmay also be of "metal and.comprise the other electrode, such cover and body being separated by a gasketor. spacer of insulating material, such as wood, rubberor'th'e like. One or both of such electrodes. may "be made wholly of zinc or cadmium, ormay; beLmade ofany'desired'metal with a' surface coating of zinc or cadmium" on the' face'of' the elec* trode which contacts thefoam'.

The function of "the electrodes, in the low fre-.' quency balanced alternating current heating of the latex'froth of:therpresentdnvention; is very difi'erent' fromzthatz'in: the. deposition 0f'S01idiIl1b-'- ber masses ion electrodes; in theso-called: electro deposition of rubber from" liquid: latex by l the use of direct 'electriclicurrent onunbalanced alternat--' lug-current: Also; of course;.the low frequency: electric: current heating: to; which the present i inventionrelates; wherezthe: lowfrequency 1 electric" current for heating thel'foam isvcarriedcthrough the-foam, .is very different fDGHIJthBiSOECaIISdThiQH frequency dielectric: heatingi Where the. heat. is oreatedsin the latex foam asiancelectric insulator wh-iclmis placed-in: a: varying electrostatic field-Jot. higlrfrequency; suchas radio-frequency.

The latex-for preparing: sponge rubbberaccordxingctorthe present invention may 'be a natural: rubberdatex; or 1 aconjugated 'diene polymer synthetic rubber. latex; or'mixtures-of any ofithesame. Such-conjugated: di'ene polymer synthetic: rubber: latex. may. be: an aqueous emulsion poly-' merizate ot one or: almixture of butadienes-1,3-,; for example; buta-diene-LB; 2-methyl-butadiene- 1,3: (isoprene); 2-chloro-butadiene-L3: (chloro prene), piperylena;2,3-dimethyl butadiene-1,'3.;'

The. conjugated. dienevpolymerr synthetic rubber latex;v as known, maysalso benarmixture .of 'one or. more: of? such butadienes-l ,3.-:with wone or' more: other polymerizable compounds :which are capable of formi'ng rubberyycopolymers With :butadi'enes l,3-,-.for-"example,; up tou'70%- of such mixture of one :or'more compounds which contain a. single GHZ1==G group; whereaat least one of the disconnected valences is attached to an-electro negative group; that is, a. group-which substantially increases theelectrical .dissymmetry; or-polanchar ester: of, the molecule. Examples or compounds; whiclrconta-in EttCHz=C group and are copoly merizable with hutadienes-LB are aryl olefins: SdGhiflS styrene;;and-vinyl naphthalene; thealphas methylene carboxylicacids, and-their esters, .ni-

triles and amides, such as acrylic acid, methyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, methacrylamide; methyl vinyl ether; methyl vinyl ketone; vinylidene chloride.

The latex may be compounded with the usual vulcanizing and other compounding ingredients and the froth will contain a conventional heatsetting agent which may be added to the latex before frothing, or to the froth, as before pouring into the molds. such heat-setting agent, as is well known, may be an alkali-metal silicofiuoride, or ammonium sulfate or other ammonium salt which together with zinc oxide forms a heat-setting combination. The latex compound may be converted into a froth by whipping in a beater, such as a Hobart mixer, or by the evolution of a gas in the latex, as by the decomposicarbonate and an acid in the latex in the presing the latex and preparing the froth are well known.

vention, the parts referred to therein being by weight:

Sixty-three per cent natural rubber latex was compounded according to the following formulation:

Parts by weight Rubber (in latex) 100 Potassium oleate 2.8 Sulfur 2.5 Accelerator 2.7 Zinc oxide 5 Antioxidant 1 The compounding ingredients were added as aqueous solutions or pastes in known manner,

tion of the surface which was in contact with the top plate and the cores was badly pitted. In that this surface acts as the bottom of a seat cushion, such pitting on the bottom surface of the cushion and in the tubular openings extending into the body of the cushion, was not objectionaible. Such pitting, however, would be very objectionable in the main exposed surface portion of the cushion corresponding to the bottom or pan portion of the mold.

If desired both sections of the above referred mold could be made of zinc or have the interior faces made of zinc coating, as by spraying on an tion of hydrogen peroxide or the reaction of a i 16.. ence of a stabilizer. Such methods of compound-,

aluminum mold, or dipping in case of a sheet steel mold. The body of the electrodes could be of other metals than the aluminum used, provided, of course, at least the pan portion had a p zinc coating on the interior surface of the same. The following is illustrative of the present inafter which water was added to a final concentration of 60% solids. The latex was whipped to a froth in a conventional Hobart mixer to a density of .103 grams per cubic centimeter. Four parts of potassium silicofluoride gelling agent were added at the end of the whipping operation.

The whipped froth was poured into a mold comprising a bottom metal pan 13%" x 17 and 3 /4" deep, which served as one electrode. This path was made of aluminum and the interior surface of the same was a sprayed zinc coating. A 13 x 17 1;" metal top plate was used, to which was fastened ten rows of fourteen closed end cores, each core being /2 diameter and 2 1 long, the cores projecting into the pan or mold space to form tubular openings in the finished sponge cushion in the usual manner. The top plate served as the other electrode and the electrodes comprising the top plate and bottom pan section of the mold were separated by a rubber gasket. The top plate was of aluminum, and not surfaced with zinc. The froth was heated by passing a low frequency (60 cycles, 110 volts) electric current through the same. The heating cycle was characterized by a rapid increase in current to about 16 amperes during the first three minutes. The current then fell off to 10 amperes, and after ten minutes no current flowed, the temperature of the latex foam reaching 90 C. to 100 C. in the ten minutes. After such heating the sponge had become permanently set. It was then placed in an air oven at 100 C. for 15 minutes to vulcanize the sponge, after which it was removed from the mold. The resultant cured sponge had an undistorted outer surface free from pitting and devoid of coloration, wherever the surface had been in contact with the bottom pan electrode which was zinc-sprayed. The por- As stated above, tests on stainless steel and iron show pitting and brown discoloration of the sponge surface. Copper shows very bad pitting and black discoloration. Lead shows black discoloration. Tin and nickel show yellow and black spots. Silver shows bad pitting and black spots. Chromium shows bad pitting and green discoloration. Magnesium shows bad pitting. Where the inner electrode surface is composed of cadmium, similarly to the zinc coated bottom mold section above, the surface of the sponge rubber molded against it was devoid of any surface distortion or pitting or discoloration.

In view of the many changes and modifications that may be made without departing from the principles underlying the invention, reference should be made to the appended claims for an understanding of the scope of the invention.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. The process of producing sponge rubber from a latex which comprises introducing a heatsettable latex froth into a mold, and setting the froth in the mold by passage of a low frequency alternating electric current through the froth between metal electrodes, at least one of said electrodes being in contact with a portion of the outer surface of the sponge article being molded and the surface of which electrode in contact with the foam is composed of a metal selected from the group consisting of zinc and cadmium.

2. The process of producing sponge rubber from a latex which comprises introducing a heatsettable latex froth into a mold, and setting the froth in the mold by passage of a low frequency alternating electric current through the froth between metal electrodes, at least one of said electrodes being in contact with a portion of the outer surface of the sponge article being molded and the surface of which electrode in contact with the foam is composed of zinc.

3. The process of producing sponge rubber from a latex which comprises introducing a heats'ettable latex froth into a mold, and setting the froth in the mold by passage of a low frequency alternating electric current through the froth between metal electrodes, at least one of' said electrodes being in contact with a portion of the outer surface of the sponge article being moldedand the surface of which electrode in contact with the foam is composed of cadmium.

JAMES U. MANN.

No references cited. 

1. THE PROCESS OF PRODUCING SPONGE RUBBER FROM A LATEX WHICH COMPRISES INTRODUCING A HEATSETTABLE LATEX FROTH INTO A MOLD, AND SETTING THE FROTH IN THE MOLD BY PASSAGE OF A LOW FREQUENCY ALTERNATING ELECTRIC CURRENT THROUGH THE FROTH BETWEEN METAL ELECTRODES, AT LEAST ONE OF SAID ELECTRODES BEING IN CONTACT WITH A PORTION OF THE OUTER SURFACE OF THE SPONGE ARTICLE BEING MOLDED AND THE SURFACE OF WHICH ELECTRODE IN CONTACT WITH THE FOAM IS COMPOSED OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZINC AND CADMIUM. 